1. Field of the Invention
The present invention relates in general to wavelengths measuring devices, and relates in particular to an optical wavemeter using an interferometer.
2. Description of the Related Art
An example of the structural configuration of a conventional optical wavemeter is shown in FIG. 3. The wavemeter comprises: a beam splitter 1; a fixed mirror 2; a moving mirror 3; a photoreceiver 4; a distance measuring device 5; a computation section 6; and a display section 7. For the fixed mirror 2 and the moving mirror 3, corner cube prisms, for example, may be utilized which always provides coincidence in the direction of incident/reflection light. Also, the moving mirror 3 is fixed on a linear moving stage (not shown), for example, and the linear moving stage is moved by a motor (not shown) through a belt and pulleys.
Referring to FIG. 3, the beam splitter 1 receives a target light beam 20 having an unknown wavelength X and splits the beam 20 into a reflected beam 20a and a transmitted beam 20b. The reflected beam 20a is reflected back by the fixed mirror 2 and, after passing through the beam splitter 1, is injected into a photoreceiver 4. The transmitted beam 20b is reflected by the moving mirror 3 back to the beam splitter 1, to be injected into the photoreceiver 4. The reflected beam 20a and the transmitted light 20b received by the photo-receiver 4 repeatedly generates periodic interference fringes due to the translation action of the moving mirror 3, and, through the process of photocoversion, the photoreceiver 4 generates electrical signals in accordance with the intensity of the interference fringe. A pulse signal 21 outputted by the photoreceiver 4 is received by the computation section 6 for each interference fringe.
The distance measuring device 5 measures the amount of the movement (shown by a lateral arrow in FIGS. 1, 2 and 3) of the moving mirror 3, and generates a pulse signal 22, for each resolution unit d of the distance measuring device 5, and outputs the pulse signal 22 to the computation section 6. The distance measuring device 5 utilizes known distance determining devices, based on an fringes counting method by injecting a He-Ne laser reference beam together with a target light beam into a common optical path to measure the travel distance with reference to the Intensity of the interference beam, or an opto-electronic device to measure the distance of travel of the moving mirror having a reference scale to read the distance of travel of the scale opto-electronically.
The computation section 6 calculates the estimated wavelength .lambda.m of the target light beam (of unknown wavelength) based on the count value of the pulse signal 22 outputted by the distance measuring device 5 when the pulse count of the pulse signals 21 from the photoreceiver 4 has reached a predetermined value.
The process of determination is further explained in FIG. 4 which shows the state of the pulses preceding and following a cumulative count M of the pulse signal 21. To facilitate understanding, FIG. 4 shows the rise time of the pulse signal 21 at the count M-1 to be coincident with the rise time of the pulse signal 22; however, it is not mandatory that the rise times be coincidental. Also, the explanations refer to the condition that the estimated wavelength .lambda.M of the target light beam is never higher than the true wavelength .lambda. of the target light beam (i.e. 0.ltoreq..epsilon.M&lt;1), where m is a fractional count of a pulse (shortened to fraction, hereinbelow); however, it is not necessary in practice that this condition be met.
In FIG. 4, it is shown that the magnitude of the fraction .epsilon.M increases successively from .epsilon.1d , 2.epsilon.1d to 3.epsilon.1d, however, according to the apparatus shown in FIG. 3, the true value of .epsilon.M is unknown, because the count M of the pulse signal 21 from the photoreceiver 4 is a pre-determined value, and relates only to the corresponding cumulative count N(M) of the pulse signal 22 from the distance measuring device 5.
Furthermore, a count value n(M) of the pulse signal 22 for one fringe, generated by the distance measuring device 5, is given by n(M)=N(M)-N(M-1) which contains a fractional information, but this quantity is not measured in the conventional apparatus.
The operation of the conventional computation section 6, shown in FIG. 3, will be explained with reference to FIG. 4. The pulse signal 21 is generated for each fringe (in other words, every time the moving mirror 3 moves a spacing of half a wavelength, .lambda./2), and the pulse signal 22 is generated for each travel distance d of the moving mirror. The travel distance d is equal to the resolution capability of the distance measuring device 5. Here, if it is assumed that counting is commenced from the rise time of both pulse signals 21 and 22, and letting M be the count value of the pulse signal 21 and N be the corresponding count value of the pulse signal 22, then the following equation holds: EQU M.lambda./2=d(N(M)+.epsilon.M) (1)
where .epsilon.M is a fraction within a range 0.ltoreq..epsilon.M&lt;1. In practice, the circuit count is triggered by the rise event of the pulse signals, therefore, the count value includes the fractional part, and the count value of the pulse signal 22 becomes N(M)+1; however, in equation (1), N(M) is obtained by subtracting 1 from the count value N(M)+1.
In the optical circuit configuration shown in FIG. 3, the computation section 6 calculates the estimated wavelength .lambda.M from the pulse data shown in FIG. 4 using the equation (1) according to the following equation (2): EQU .lambda.M=2dN(M)/M (2)
The wavelength error .DELTA. is also obtained from equation (1) as: EQU .DELTA.=2d.epsilon.M/M (3)
however, because the fraction .epsilon.M is an unknown, and in practice, it is rounded-off and specified as: EQU .DELTA.&lt;2d/M (4)
As explained above, to measure the (true) wavelength .lambda. of a target light beam using the conventional wavemeter, the wavelength error is specified under the worst condition given by equation (4) because the fraction .epsilon.M is not measured. However, in general, the value of the wavelength .lambda.M obtained by computation is often much closer to the value of .lambda. than those given by equation (4). Furthermore, using the fringes which precede or follow the pre-determined count value M of the pulse signal 21, it is quite often possible to obtain a value close to the true value of the wavelength .lambda.. What is lacking in the exiting apparatus and method is an approach to finding an appropriate value for the fractional count M.